Microelectromechanical systems (MEMS) devices can be incorporated into a wide variety of technical applications, including sensors (e.g., accelerometers, rate gyros, tilt sensors, thermal), optics (e.g., infrared detectors, optical mirrors), and RF systems (e.g., switches, tunable capacitors, varactors, resonators, and other systems). Such MEMS devices can be made of a single material, such as silicon, polysilicon, silicon nitride, copper, aluminum, or of multiple materials, such a bilayer of polysilicon and gold or a trilayer of aluminum, oxide, and aluminum. In such multilayer applications, the multiple layers can include cladding layers, diffusion barriers, and adhesion layers.
Current wafer-level thin film encapsulation often consists of two or three layers or films. For example, FIG. 1 shows a two-layer structure. A first layer 40, a structural layer, generally consists of a dielectric material with etching vias 42 that are used to release the MEMS device 20 from within a sacrificial layer. The thickness of structural layer 40 is typically 2-4 μm. Once the sacrificial layer is removed, leaving a cavity 30, a second layer 44 is added. This second layer 44, a sealing layer, generally consists of a polymer that is used for plugging etching vias 42 after MEMS device 20 is released. For good sealing and environmental performance (i.e., hermeticity), second layer 44 will often be non-organic and hydrophobic. This layer is deposited in a vacuum chamber that also deposits some material inside the cavity and results in a very low-pressure environment within the sealed cavity.
Many MEMS applications require higher pressures for proper operation, however, than can be achieved with this method. For example and referring to FIG. 2, if initial sealing is performed at or near atmospheric pressure using spin-on organic, glass, or similar materials, a third layer 46, a barrier layer, must be added to provide the required hermeticity. This is usually a vacuum deposited layer similar to that used for the single sealing layer approach. The initial sealing layer, second layer 44, maintains the pressure inside the cavity during the deposition. Third layer 46 generally consists of a dielectric material that is used to form the hermetic seal. The thicknesses of sealing layer 44 and barrier layer 46 are typically 1-5 μm and 1.5-4 μm, respectively. It is noted that in FIG. 2, MEMS device 20 is fabricated in a cavity on the device wafer. The cavity can be formed by etching a substrate 10 or depositing materials (e.g., silicon, oxide, glass) around MEMS device 20 followed by Chemical-Mechanical Planarization (CMP).
Because the film deposition is conducted at elevated temperatures and the thin film encapsulation is heterogeneous in nature, extrinsic thermal stress will typically be generated in the films after the film deposition process. The stresses are a function of geometry (e.g., length, thickness), material properties (e.g., Young's modulus, Poisson's ratio, and CTE), and processing conditions (e.g., temperatures, pressures). These stresses can occur at high levels in the as-deposited thin film encapsulation, which can cause defects in the film, such as delamination, cracks, moisture absorption, or the like. In particular, high tensile thermal stress often occurs at the edge of the third dielectric layer, which can result in cracks. In fact, finite element analyses have shown that a maximum tensile stress can be around 460 MPa, which is significantly large. This thermal stress is induced by the CTE mismatch between the third dielectric layer and the second polymer layer deposited/patterned outside the perimeter of the encapsulation. Further, if the sealing material is a polymer, it can adsorb moisture through defects (e.g., cracks, pores) in the barrier layer, which can result in degradation of the polymer and water vapor transmission to the MEMS devices. Accordingly, improved designs and methods for wafer-level thin film encapsulation for MEMS packaging would be desirable to minimize or eliminate the detrimental effects caused by these stresses.